Detection of pregnancy in cattle, especially dairy cattle, is of great economic importance. Production of the hormone moiety required for milk production depends on a routine regimen of pregnancy for each cow. Toward this goal, and for a number of other reasons, 70% of the dairy cows in the U.S. are artificially inseminated. Conception can only occur if insemination occurs at the proper stage of the estrous cycle of the cow. Since this receptive phase of the estrous cycle lasts only a relatively short time, an unsuccessfully bred cow can not be reinseminated until estrus (heat) occurs in the following cycle (21 days later). An important objective of the dairy industry, therefore, is to detect those animals in which a successful pregnancy was not established so that they may be rebred at the next cycle. If detection occurs before the first cycle is complete, the cow can be reinseminated in the next heat, about 21 days after the first attempt. If detection is not possible in that time reinsemination must wait until the second cycle, 42 days after the initial insemination or the third cycle at 63 days. The economic impact of having non-pregnant cattle for these extended periods of time can be substantial.
Presently, the majority of pregnancy detection in cattle is performed by veterinarians. The method used is a rectal palpation where the vet physically feels the presence of a developing fetus by "slipping" or manipulating the membranes of the placenta. This technique only becomes effective, however, in the period between 30 and 42 days of pregnancy. Depending on where in that time interval detection is made, at least one, and perhaps two cycles could be missed. There are other problems with the rectal palpation method: first, the vet has to be present at the farm; and second, while the data are inconclusive, it has been reported that there is a perception on the part of some farmers that the mechanical manipulation required with rectal palpation might increase the risk of spontaneous abortion.
Another method of pregnancy detection is the assay of progesterone in milk. Progesterone is a steroid which is necessary for normal development and maintenance of pregnancy. Serum titers of progesterone rise throughout early stages of pregnancy, and increased levels of the hormone are found in milk. A radioimmunoassay (RIA) can detect the increase in milk progesterone. This technique has a number of shortcomings. By definition, an RIA requires the use of radioactive material. Inherent in this requirement are several disadvantages, including the need for sophisticated laboratory equipment, preventing both the farmer and most veterinarians from performing the test. In addition, the level of progesterone varies both between normal non-pregnant cows and within individual cows throughout the estrous cycle. Therefore, baseline readings for each cow are necessary, and at least two determinations are required to confirm pregnancy. Furthermore, this method is reported to be unreliable earlier than 20 days after insemination.
It is clear that a more simple, rapid and accurate means of detecting pregnancy in cattle would be of great value.
The present inventors have reported, J. Animal Science, 57, Supp. 1, 320 (1983), the initial isolation and partial characterization of protein specific to the chorionic membrane of bovine embryos. This protein was reported to be a high molecular weight compound which, at one point in the isolation process, appeared to stain as a glycoprotein. However, this protein had not been fully structurally characterized or sufficiently purified to be utilized for practical purposes. Furthermore, it had not been reported or suggested that such a protein would be found in the body components of the mother or would be diagnostic for pregnancy, or provide the basis for a pregnancy assay.
It has not been hitherto known that a bovine pregnancy antigen can be isolated from the embryo or body components of pregnant cattle, or that such an antigen could serve as the basis for a diagnostic pregnancy assay.